As part of the background description, the following terms are to be briefly explained:
STP—Spanning Tree Protocol (including its various standard variations such as STP—IEEE 802.1D, RSTP-802.1w, or MSTP-802.1s), is standard for protection and loop prevention in Ethernet communication networks. The STP recalculates the active topology in case of faults or reconfiguration. The STP also determines the active topology (STP network topology) to carry traffic upon initialization. According to STP, any network can logically be represented in the form of one or more interconnected trees—STP trees. An Ethernet network may be covered by a single STP tree; one STP tree may be provided per one VLAN; one STP tree may cover a group of VLANs.
VLAN—Virtual Local Area Network forming a logical partitioning of an Ethernet network (e.g. as defined per IEEE 802.Q) and defining a broadcast domain, within the Ethernet network;
GVRP—Generic VLAN Registration Protocol—Protocol for dynamically registering/provisioning or deregistering VLANs on a per VLAN basis.
Switching node—(or Ethernet switch or bridge) a network node having a number of ports for receiving and transmitting data frames.
Forwarding port—a port of a switching node being presently active, according to a presently actual STP topology, to receive data frames and forward them to one or more other ports of this or another node.
Ports assigned to a particular VLAN—ports of a switching node which, according to a selected service, are predestined for transmitting data packets to/from the particular VLAN.
Forwarding port—a port of a switching node
Edge port—a port that is on the boundary of an STP/VLAN domain; the edge port is considered a permanently forwarding port.
FDB—Filtering database—a standard database of MAC addresses in an Ethernet switching node, which is self-learned i.e., gradually formed during operation and based on registering source addresses from which data frames arrive to particular ports, thereby FDB “learns” the network for facilitating further switching decisions.
PDU—(Protocol Data Unit) a management message in any of the concerned protocols such as STP, GVRP, or the proposed below technique/protocol.
Spanning Tree Protocol (STP) is widely used for loop prevention and protection in Ethernet switched networks. Ethernet switches (nodes) forming ring or mesh networks might be connected to Local Area Networks (LANs) or end stations. A LAN may also comprise a number of Ethernet switches, where some support terminal/access ports (i.e. those directly connected to a LAN end-station or to those on the boundary of one STP domain to another). For increasing the scalability of Ethernet switched networks, also utilized are so-called Virtual Local Area Networks (VLANs). A number of VLANs are usually defined in the network.
VLANs may overlap one another, and normally, each particular VLAN must span each Ethernet switch that may be encountered in a path from one terminal to another, in any possible topology that may be imposed by the STP. Such a configuration of VLANs will be further called initial VLANs configuration. In many cases it can be that the initial configuration requires that all VLANs are provisioned on all ports throughout the network that are not edge ports. VLANs can be configured on the Ethernet network by GVRP or a management interface such as SNMP.
U.S. Pat. No. 6,515,969 relates to a method and apparatus for disseminating Virtual Local Area Network (VLAN) membership information across computer networks defining multiple spanning trees, which is hereby incorporated by reference. An intermediate network device includes a plurality of ports and a plurality of spanning tree engines each associated with one or more VLAN designations defined within the network. The spanning tree engines transition the ports among a plurality of port states, including a forwarding state and a blocked state. For each port, a separate Generic Attribute Registration Protocol (GARP) participant is also established and each GARP participant includes a multiple spanning tree (MST) GARP VLAN Registration Protocol (MST-GVRP) application component and an associated GARP Information Declaration (GID) component. The MST-GVRP application components cooperate to define a plurality of GARP Information Propagation (GIP) contexts each of which is associated with a spanning tree engine and thus its one or more VLAN designations. The technology described in the U.S. Pat. No. 6,515,969 relates to establishing the network topology whenever required, no optimization or special provisions for topology change and transition periods are mentioned. Also, the patent does not describe any solution for reducing broadcast traffic within a VLAN.
For addressing a particular data packet (frame) from a source node to a destination node, protocol for Ethernet networks supporting VLANs (IEEE 802.Q) requires that the data frame carry indication of a specific VLAN to which both the source and the destination node(s) belongs. According to the basic approach, Ethernet switches, passing there-through a data packet which is not addressed to a known MAC address but carries indication of a specific VLAN, performs “flooding” of all its output ports assigned to that VLAN. Thus, all the relevant neighboring nodes are enabled to continue checking and forwarding that packet for the specified VLAN and, if indicated and known within the switch, to the specified destination MAC address.
As mentioned, any Ethernet switch supports the Filtering Database (FDB) which is gradually formed by recording the source address indicated in a data packet (i.e. Ethernet frame) when the frame is received by one of the ports. A frame arriving to the switch and destined to a specific unicast MAC address that exists in the FDB, is not broadcasted to all output ports of the node, but is switched to a particular port as presently known to the switch with respect to that address. Therefore, the FDB allows reducing excessive flooding of traffic in the network.
It should be noted that an STP protocol, when applied to Ethernet switched network, creates a so-called “logical cut” in each cycle/ring, to impose an active tree topology. Consequently, all FDB information must take into account the given STP tree structure. If a fault occurs in any span of the structure, the STP protocol urgently reacts to that and recalculates the tree structure(s). The recalculation of the Spanning Tree results in a different network topology which, of course, will dictate other switched paths between those source nodes and destination nodes which are usually in communication. Significant portions of the FDBs supported by the switches associated with the changed STP tree topology are deleted as soon as the topology is changed. The changes in the STP topology cannot be processed immediately so, at the beginning, a huge amount of traffic is broadcasted or flooded at the switching nodes of the network (since a significant part of the FDB or the complete information previously recorded by FDB is deleted and new information is not yet collected).